1. Field of the Invention
The invention relates to an secondary electrochemical cell with at least one positive electrode which comprises an active material and one negative electrode, which are held directly in a housing which has at least one detector element or which is dynamically connected to one such detector element. The latter being designed or can be set to acquire a predetermined unallowable operating state of the secondary electrochemical cell and in an unallowable operating state of the secondary electrochemical cell to actuate at least one switching element which prevents recharging and/or discharging of the secondary electrochemical cell.
2. Description of the Related Art
In secondary electrochemical cells a pressure rise in the interior of the housing of the secondary electrochemical cell can occur, for example, upon excess charging or discharging with an unallowably high current and it leads to deformation of the housing which can become so large that chemicals, especially in gaseous or liquid form, emerge from the housing in large amounts. Various protective mechanisms have been proposed to increase the operating safety of secondary electrochemical cells.
Thus, publication EP-A-0 470 726 discloses a secondary electrochemical cell which has a cylindrical housing with a pressure membrane as the detector element. The housing directly accommodates the electrodes of the secondary electrochemical cell, the pressure membrane as part of the housing being integrated on the face side in the housing and bulging when the pressure rises in the interior of the housing, so that a plate-shaped switching element which is centrally connected to the pressure membrane reversibly or irreversibly interrupts the electrical contact between the electrode and a terminal contact which is provided on the outside of the housing as soon as the pressure within the housing reaches a predetermined value.
EP-A-0 322 112, EP-A-0 360 395 and EP-A-0 370 634 disclose providing electrochemical cells with a switching element which, when a certain still tolerable boundary deformation of the housing is exceeded, preferably irreversibly separates an electrical terminal contact from a pertinent electrochemically active electrode in order to prevent further deformation of the housing. The housing directly accommodates the electrodes and surrounds an electrically conductive cylindrical housing section which is closed on the face side and which makes contact with an electrode, on the end face the plate-shaped switching element being attached centrally by means of an electrically insulating cement from the outside. The electrically, likewise, conductive switching element in its base position, via its outside edge, forms an electrical connection between the housing section and the electrical terminal contact which projects to the outside and which is located in the center of the switching element. When the pressure rises within the housing the face side of the housing section acting as the detector element bulges to the outside, by which the contact between the housing section and the outside edge of the switching element and, thus, between the one electrode and the terminal contact is interrupted. One typical application of these switching elements is standard D cells. The bulging of the face side, starting from which the switching element interrupts electrical contact, in this case is 0.76 mm to 1.8 mm. For bulging of more than 1.8 mm emergence of chemicals from within the housing can ordinarily be expected.
EP-A-0 674 351 discloses a secondary electrochemical cell with a housing which comprises a cutting device which can be actuated by a pressure membrane and which, when a boundary pressure is exceeded within the housing, irreversibly, severs an electrical conductor which connects the terminal contact of the secondary electrochemical cell to an electrochemically active electrode.
Also, when using a switching element which interrupts the electrical connection between the terminal contact and the pertinent electrochemically active electrode when a certain pressure within the housing is exceeded, it is possible for the pressure to continue to rise and ultimately for chemicals to emerge from the housing of the electrochemical cell or for the latter even to explode. For this reason, for example in EP-A-0 364 995, EP-A-0 573 998 or EP-A-0 739 047 it is proposed that a pressure membrane which actuates the switching element and which is integrated into the housing be provided with a bursting area via which after activation of the switching element and a further pressure rise chemicals can emerge from the interior of the housing.